2009 Pontiac G6 3.5L Thermostat Replacement (by removing headlamp)

1. Remove Passenger Headlamp (7mm socket and plastic fastener removal)
2. Remove metal clip that holds connector in place. (Needle nose pliers)
3. Remove two 10mm bolts to access thermostat.
4. Replace thermostat.
Put everything back together.
5. Add coolant to overflow reservoir.
6. If coolant does not drain out of reservoir back into the engine, check to see if there is a blockage in the vent line where it attaches to the top of the reservoir. My vent line was clogged in plastic tube where the rubber hose attaches to the tank. The inside diameter of the plastic tube is very small (about 1-2 mm) and clogged easily. If air cant flow back to the tank through the vent line (from the top of the engine), then it is very difficult to get all the air out.
I used a very small allen wrench (or paper clip) to poke the gunk towards the reservoir where I held a small rag to catch it. Be careful if the coolant is hot.
On my first try, I did not get all the air out of the lines and the temperature cage quickly climbed up to 250 F. It should have stayed at the thermostat opening value of 185 F.